broken on d3 v4.

[ensemblebd]

d3.layout.tree is now d3.tree
http://stackoverflow.com/questions/38337997/d3-js-v4-d3-layout-tree-has-been-removed

So with that fixed, the [tree].nodes is non functional. Looks like it's using a new thing called d3.heirarchy.
https://github.com/d3/d3/blob/master/CHANGES.md#hierarchies-d3-hierarchy

So yeah, guess we'll need to stick with version 3 for now. Looks like a pretty hefty update.